Background <p>Exercising with others has been shown to yield mental health benefits, particularly among older adults. However, evidence regarding physical health outcomes and younger populations in Japan remains limited. This study aimed to examine whether exercising with others is associated with better physical and subjective health among Japanese adults and to assess the mediating roles of social interaction and exercise habits.</p> Methods <p>This study analysed cross-sectional data from 28,680 Japanese adults (mean age = 50.2&#xa0;years) who participated in the 2022 nationwide sports participation survey. Participants were classified into four groups: non-exercising, exercising alone, exercising with others, and mixed-type (both alone and with others). Logistic regression was used to assess associations between exercise arrangement and health outcomes including underweight (body mass index &lt; 18.5&#xa0;kg/m²), obesity (body mass index ≥ 25&#xa0;kg/m²), and subjective health, adjusting for demographic variables. Mediation analyses evaluated indirect effects via social interaction and exercise habits.</p> Results <p>Compared with exercising alone, exercising with others was associated with lower odds of being underweight and higher odds of reporting good subjective health. Similar trends were observed for mixed-type exercisers. No significant association was found with obesity. Mediation analyses revealed small but significant indirect effects on subjective health through social interaction, while direct effects remained substantial.</p> Conclusions <p>Exercising with others is linked to improved subjective health and lower odds of being underweight among Japanese adults, partially mediated by social and behavioural factors. Facilitating opportunities for group-based exercise may be relevant for health promotion in Japan and should be evaluated in longitudinal and intervention studies.</p>